1. Field of the Invention
The present invention relates to a power system stabilizer (hereinafter to be briefly called PSS) and more particularly to a PSS for use in an excitation system for a synchronous motor.
2. Description of the Prior Art
FIG. 1 is a block diagram of a general excitation system including a prior art PSS as disclosed, for example, in Japanese Patent Publication No. 53-44204.
Referring to the figure, 1 denotes an input terminal of a deviation of terminal voltage of an electric motor from its reference voltage, 2 denotes a PSS, 3 denotes an input terminal of the PSS 2, 4 denotes a damping circuit, 5 denotes an adder for deducting the output of the damping circuit 4 from the sum total of the deviation from the input terminal 1 and the output of the PSS 2, 6 denotes a regulator for controlling an exciter 7 based upon the output of the adder circuit 5, and 7 denotes the exciter controlled by the regulator 6 and supplying field voltage to the motor (not shown). Denoted by 2a is a filter circuit for determining the response range of the PSS 2 to the input signal 3, which generally has a transfer function characteristic expressed as ##EQU1## Denoted by 2b is a portion for compensating for time-delay of the regulator 6, exciter 7, motor, and the like and is a lead-lag circuit expressed generally by the form of Denoted by 2c is a limiter for limiting the output signal of the PSS 2 so that it has a suitable signal level for the performance of the excitation system shown in FIG. 1 as a whole.
As the input signal to the PSS 2, the deviation in the number of revolutions of the rotor of the motor, deviation in frequency of the terminal voltage of the motor, deviation in the output power of the motor, and the like is generally used.
Operation will be described below. When the motor terminal voltage deviates from the reference value, a deviation signal is applied to the input terminal 1, and this deviation signal is amplified in the regulator 6 and input to the exciter 7. The signal is further amplified in the exciter and supplied to the field of the motor, whereby the deviation of the motor terminal voltage from the reference value is controlled to be returned to zero. The damping circuit 4 is provided for stabilizing the aforesaid voltage control. While the above is the function of what is called general automatic voltage regulator (hereinafter to be referred to as AVR), the PSS 2 is a control apparatus for providing the adder 5 of the aforesaid excitation system with a properly amplified and compensated auxiliary signal (for example, deviation in number of revolutions of the rotor of the motor) thereby to improve stability of the power system.
Now, let us think about a PSS taking in the number of revolutions of the rotor of a motor as input thereto. Detected deviation in the number of revolutions is applied to the input terminal 3 of the PSS. The signal is made free of D.C. component and high frequency component in the filter 2a and applied to the compensating circuit 2b so as to be properly amplified and compensated for its phase. The signal is then limited in the limiter 2c to a level not exceeding a suitable signal level for the excitation system and applied to the adder 5, whereby the output voltage of the exciter 7 is controlled so that the power swing of the rotor of the motor is suppressed.
The working principle of the PSS will be described below. FIG. 2 is a block diagram showing linear approximation of fluctuation in a motor in a single-machine infinite bus system as described, for example, in "THE SOCIETY OF ELECTRICAL COOPERATIVE RESEARCH", Vol. 34, No. 5. Referring to the figure, K.sub.1 represents a coefficient of synchronizing torque produced by a motor whose field flux linkage is constant, K.sub.1 ' represents a coefficient of synchronizing torque produced by the AVR, K.sub.1 " represents a coefficient of synchronizing torque produced by the PSS, D represents a coefficient of damping torque produced by the motor whose field flux linkage is constant, D' represents a coefficient of damping torque produced by the AVR, and D" represents a coefficient of damping torque produced by the PSS. Generally, when the phase angle .theta. becomes larger where the power factor is close to 1.0, the coefficient D' is liable to take a negative value. In the case of an AVR with high response and high gain, in particular, D +D' sometimes becomes negative, whereby the system fails to keep steady-state stability on account of lack of damping power. In such a case, stability is attained by having the PSS additionally provided to produce the damping force D". FIG. 3 is an explanatory diagram showing such behavior. To cancel the negative damping force D' produced by an AVR having high response and high gain and thereby to improve the condition of damping force, the PSS produces damping force D" acting in opposite direction to the negative damping force. However, the PSS has no object to improve the synchronizing force, and therefore, K.sub.1 " sometimes becomes very small or, in some case, takes a small negative value.
While the prior art PSS is organized as described above, since there is existent only one mode of power fluctuation in the single-machine infinite bus system (or a system similar to it) as is the case with the above described prior art, a good result is obtained by properly executing the phase compensation of the PSS against the one mode of power swing. In an actual power system, however, there are many cases where simulating the one-machine infinite bus system is difficult. In a power system simulating double-machine infinite bus system, for example, there are existent two modes of power swing. FIG. 4 shows a waveform of power swings in which two modes are present. In the case of the two modes of power swings, the phase compensating values of the PSS in the first power swing mode and in the second power swing mode, that is, the constants in the above mentioned lead-lag circuit, do not always become equal but normally the optimum compensating values are different from each other.
Therefore, even if the phase compensation is made most suited for the first power swing mode, correct phase compensation is not provided for the second power swing mode, and as a result sufficient effects of the PSS are not obtained. Further, since the component of the first power swing mode and that of the second power swing mode contained in a power swing resulting from an external disturbance (line fault) of a power system become different depending upon disturbances, it is not expectable that most suitable phase compensation is achieved against every external disturbance (line fault). Since there have been such problems sufficient PSS effects have not been obtained.